Electronic devices, such as transistors, diodes, resistors, capacitors, transducers, monolithic packages, hybrid microcircuits, contain one or several dies or chips bonded onto a rigid substrate with an adhesive.
The die is a semiconductor on which all the active and passive microcircuit elements have been fabricated using one or all of the techniques of diffusion, passivation, masking, photoresist and epitaxial growth. They are usually made of doped Silicon or Gallium arsenide (Gas) and cut out with a diamond saw from wafer discs into individual dies.
The substrate provides mechanical support but may also serve useful thermal and electrical functions. It may be a ceramic, glass, sapphire, metal, quartz, epoxy, polyimide, etc. They may be bonded to a larger, rigid plate, board or case usually made of metal, ceramic, glass or high temperature plastic, providing protection, mechanical support and hermeticity. Alternatively, the substrate itself may provide, by its design, these functions.
Dies are usually fixed to the substrate with an epoxy adhesive. It is often necessary to provide thermal and electrical conductivity to the adhesive bond thus formed by the use of conductive adhesives. These are currently available as B-stage epoxy sheets coated on both sides of glass fabric for support. The epoxy adhesive presents a great number of disadvantages:
Since the epoxy is of a tacky, semi-solid consistency, it must have a glass fabric carrier in the middle and a release-coated plastic film on both sides of the sheet; PA1 It requires freezing or refrigeration for storage; PA1 It has to be cured or polymerized for 1-24 hours at 90.degree.-200.degree.; PA1 It usually contains large quantities of ionic impurities, often up to 400 ppm Cl.sup.- and Na.sup.+, which are conducive to corrosivity and hence decreased reliability of the circuit; PA1 Once the die is bonded with the epoxy thermosetting adhesive, it is very difficult to remove them it if rework is required for some reason. PA1 The adhesive is sandwiched between the die and the substrate or other support; PA1 The film is heated to its melt and pressed between the preheated die and substrate for 0.001-60 minutes; PA1 The pressure is released and the bonded assembly is cooled to ambient temperature. PA1 The adhesive is self-supporting, non-tacky solid film needing no glass fabric support and release liner; PA1 It has long shelf life at room temperature requiring no refrigeration; PA1 Only short processing times are necessary since there is no long reaction type curing; PA1 The adhesive of the present invention has generally considerably better ionic purity then epoxies; PA1 Because of the adhesive's thermoplastic nature reworking is easy. One has only to apply heat and remove the die with a pair of tweezers. PA1 Sulfone polymers, (PSO) , such as polysulfone, polyethersulfone, polyarylsulfone, and modified polysulfone. Many of these polymers may be represented by the formulas of their repeating units--O--C.sub.6 H.sub.4 --SO.sub.2 --C.sub.6 H.sub.4 -- and C.sub.6 H.sub.4 --SO.sub.2 --C.sub.6 H.sub.4 --O--C.sub.6 H.sub.4 --C(CH.sub.3).sub.2 --C.sub.6 H.sub.4 -- PA1 Polycarbonates, (PCT), such as --COO--C.sub.6 H.sub.4 --C (CH.sub.3).sub.2 --C.sub.6 H.sub.4 --O-- PA1 Polyesters, such as polybutylenetherepthalate (PBT), aliphatic polyesters (APES); PA1 Polyetherimides, (PEI), such as ##STR1## Polyarylates, (PAR), such as the bisphenol A ester of terephthalic acid; Polyvinylidene fluoride (PVDF); PA1 Copolymers of ethylene (ECOP) with minor amounts (less than about 1/3) of maleic anhydride, methyl or ethyl acrylate, 10-35 weight percent vinyl acetate, or 3-10 weight percent of a carboxylic acid containing monomer optionally neutralized with a metal. PA1 Polyamides of nylons; PA1 Cellulose esters such as cellulose acetate, propionate, nitrate, and ethers, such as ethyl cellulose (CELL); PA1 Linear phenoxy resins such as (CH.sub.2 --CHOH--CH.sub.2 --O--Ar--O--).sub.n ; PA1 Acrylic polymers (PAC), which are homo or block copolymers of alkyl esters of acrylic and methacrylic acid, optionally with up to 10% of acrylic and methacrylic acid and up to 305 styrene or vinyl acetate; PA1 Polyurethanes, (PUR), prepared from diisocyanates and glycols, such as polyesterglycols or polyetherglycols. PA1 Antifoam, 0-0.5 weight percent, based on solvent; PA1 Wetting agent, 0-2 weight percent, based on filler; PA1 Filler of this invention, 0-45 volume percent of the adhesive; PA1 Antioxidant, 0-2 weight percent of the polymer. PA1 Positioning the adhesive film between the die and substrate. More than one layer of film may be used and more than one die may be bonded to a substrate. PA1 Heating the film to its melt and pressing it between the die and substrate. Heat may be applied with a hot plate, oven, or induction heating. The temperature of the molten film depends on the polymer, but should be, in general, at least 50.degree. C. higher than the Vicat softening temperature such as to obtain good flow to wet the adhering surfaces. PA1 Pressure to the die or substrate may be applied by hand or a machine with scrubbing action. The pressure ought to be great enough and the time for pressing long enough for the adhesive to contact and wet the bonding surfaces without squeezing the adhesive out of the bond line. The higher the pressure, the lower the temperature may be for bonding, for less time. Useful pressure and time parameters are 0.0-100 pounds per square inch and 0.001 seconds to 60 minutes. If the time and temperature are adequate the weight of the die itself may be sufficient pressure. PA1 The pressure is then released and the bonded assembly is cooled to ambient temperature to form a strong bond between the substrate and its support.
The object of this invention is to overcome these difficulties.